The alternating movement of drawing carriages requires exactly controlling the path over which the wire travels through the drawing machine. It becomes obvious that the carriages need different speeds during the working phase and their return movement to assure constant and continuous advance of the wire throughout its whole length with overlapping phases of traction of the two drawing carriages. To guarantee a constant drawing speed, each drawing carriage first has to be accelerated in the drawing direction before its clamping jaws may grip the wire and take over the tractive force. To make sure, that the wire is drawn through the die at a constant speed, the working phases of both carriages slightly overlap. The grip of the pair of clamping jaws has to be released again, before the carriage reaches the end of its path. For the rest of the path the carriage has to be slowed down and thereafter brought back to its starting position. An exact timing for these different functions is necessary to extend the working range of the path of each carriage to the maximum possible.
Wedge-shaped clamping jaws are often used to grip the wire. They are slideably mounted in forwardly diverging guideways. To reduce the friction between the guideways and the jaws, the latter usually are backed up by a series of rollers on the guideways. The jaws are moved axially in their guideways to clamp the wire radially. Once the wire is gripped by the jaws and is kept under axial force the gripping mechanism needs no further actuation and clamps the wire as long as the axial force is exerted on the wire. The clamping mechanism opens as soon as the axial force is switched off. A continuous drawing machine is known from the German published patent application No. 28 06 380, having two drawing carriages with wedge-shaped clamping jaws. To control the movement of the clamping jaws a shaft is connected to the main drive of the drawing machine and bears a cam for each carriage. The shaft is rotatably supported on the machine frame. The cams being located in close proximity to the starting position of each carriage. An arm with a cam follower is rotatably mounted on each carriage, the cam getting in touch with the lever as soon as the carriage leaves its starting position, thereby axially moving the jaws and clamping the wire. That is the moment, when the carriage under consideration has finished its acceleration and the other carriage begins to slow down. In this instance the carriage under consideration takes over the tractive force needed for the drawing operation and the automatic clamping effect is achieved in the carriage under consideration.
The immense precision and the speed of actuating the clamping jaws is not sufficient in the known drawing machine because of the fact, that the lever protruding from the carriage and controlling the clamping action of the jaws, may not touch the cam, when approaching the starting position of the carriage, i.e. on the return of the carriage but only when it moves in the drawing direction in the aforementioned instant, when the drawing carriage has to take over the tracting force. So the stationary cam has to overtake the starting carriage and the lever mounted thereon. This control mechanism is very expensive and difficult to manufacture and to adjust.